GB2520920A

GB2520920A - Beam scanning antenna

Info

Publication number: GB2520920A
Application number: GB1317997.3A
Authority: GB
Inventors: Francisco Javier Vazquez Sanchez
Original assignee: Cobham CTS Ltd
Current assignee: Chelton CTS Ltd
Priority date: 2013-10-11
Filing date: 2013-10-11
Publication date: 2015-06-10
Anticipated expiration: 2033-10-11
Also published as: GB2520920B; GB201317997D0

Abstract

A beam scanning antenna for generating and aiming a directive radiation beam comprises a waveguide with a controlled artificial dielectric structure. The waveguide comprises periodically distributed capacitors and a fixed metallic grating layer on the walls of the waveguide. The artificial dielectric is connected to the grating layer by plural via holes and respective electronic switching devices are aligned with respective apertures in the grating layer such that, in use, the dielectric constant of the artificial dielectric layer is controlled by the switches. This changes the propagation constant for the guided wave which interacts with the apertures to produce a directive beam which can be scanned in the plane of the propagation of the guided wave. Switches such as PIN diodes, MEMS, tunable resonators or optical switches may be used to control the embedded capacitors for varying the dielectric constant of the artificial dielectric structure.

Description

Beam Scanning Antenna The present invention relates to a beam scanning antenna.

Radio services often require antennas able to produce a narrow directive beam which can be pointed over a range of directions ensuring optimum use of radiofrequency power and discrimination from unwanted sources. Antennas which can steer the beam by electronic means are highly desirable, as mechanical pointing of the aperture has obvious limitations in terms of size, scan speed and quite often reliability.

Phased array antennas are the classical solution to beam steering by electronic means. This kind of antenna makes use of control elements to modify the phase of the signal received or transmitted by each element of an array of radiators to shape the overall phase front radiated from the antenna. This type of antenna can take one of a number of forms depending in the configuration of the radiating array elements (i.e. planar, conformal, type of lattice) and the architecture and technology of the phase control element (i.e. electronic analogue or digital, electro-optical).

Phased arrays able to scan the beam in any direction require placing of control elements in each element of the array. In order to compensate for the loss of the phase control elements, amplifiers need to be added, increasing the complexity and cost per array element. As array elements need to be spaced at less than half wavelength at the frequency of operation, the number of controls required in a high gain directive antenna is in the region of thousands or even tens of thousands. Given that this large number of control elements needs to be integrated within the antenna it is generally only affordable for high value applications where the scan speed is essential (i.e. radar), however phased array solutions are typically excluded from communications applications (i.e. satellite) where it is required the user terminal to be relatively low cost.

Other alternative technologies have been using the interference of a guided wave with a periodic metallic grating structure to produce a steerable radiation beam. The alteration of the propagation constant of the guided wave by mechanical means allows the scanning of a beam over certain range of directions keeping the antenna flat, minimizing the volume required by the antenna. In an alternative configuration, changing the grating pitch and orientation, it is possible to scan a beam produced by interference with a guided wave (i.e. parallel plate). These technologies, despite reducing the size and maintaining a low profile of the antenna, still have some of the limitations of mechanical systems, such as low scan speed. Furthermore, such designs cannot usually be arranged to operate easily over multiple frequency ranges, which means that they cannot be used for both transmission and reception for most applications where transmission and reception often use separate frequency bands. This means that multiple antennas need to be provided in such circumstances or the antenna has to have two regions, one for each of transmission and reception, doubling its size.

Accordingly there is a need for a beam scanning antenna that can be produced at low cost and yet which has high scan speed and is compact. The antenna according to the invention seeks to provide this.

According to the present invention there is provided a beam scanning antenna arranged to generate and aim a directive radiation beam, the antenna comprising: a waveguide comprising a controlled artificial dielectric structure comprising periodically distributed capacitors placed on a conductive wall of the waveguide; a fixed metallic grating layer on the wall of the waveguide; wherein the artificial dielectric is connected to the grating layer by plural via holes and respective electronic switching devices aligned with respective apertures in the grating layer such that, in use, the dielectric constant of the artificial dielectric layer can be controlled by the switches to control the beam direction.

The present invention is a new type of antenna which is able to electronically form and scan a directive radiation beam using a guided wave which interacts with a fixed metal grating screen. The direction of the beam is defined by the propagation constant of the guided waves in orthogonal axis and the periodicity of the grating.

The propagation constant for the guided wave is varied by an electronically controlled artificial dielectric whose effective dielectric constant is changed by switching a number of embedded capacitors altering the capacitance per unit of volume of the structure. This artificial dielectric enables operation of the antenna at high scan speeds whilst reducing cost and antenna size.

The embedded capacitors can be integrated in the structure by usual printed circuit board (PCB) manufacturing techniques and are typically switched by low cost PIN diodes. Other switching technologies like MEMS or based on tunable resonators (i.e. using liquid crystals or ferroelectric material) can be also used to control the beam steering however.

Examples of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a side schematic view of an antenna according to the invention; Figure 2 is a side view of an artificial dielectric slab employed in the antenna of figure 1; Figure 3 is a diagram showing a PIN switch diode and loading capacitance employed in the antenna of figure 1; Figure 4 is a diagram showing a tunable resonator and loading capacitance employed in the antenna of figure 1; Figure 5 is a plan view of the artificial dielectric slab, showing via holes, planar capacitors and PIN diodes; Figure 6 is a plan perspective view of an example antenna according to the invention and based on striplines, showing a launching line and guiding lines interacting with a grating layer; and Figure 7 is a plan perspective view of an example antenna according to the invention and based on striplines, showing a dual launching line and interleaved guiding lines interacting with a grating layer for simultaneous transmit and receive operation.

Referring to figure 1, in this example the present invention comprises a slab artificial dielectric material, typically planar, which is metalized on both sides. A periodic distribution of apertures (i.e. slots) on the top layer forms a fixed conductive grating layer which interacts with electromagnetic waves propagating inside the artificial dielectric slab. This allows a wave in the material to radiate into the air. The dimensions of the lattice of the grating structure will selected so that the aperture spacing is smaller than half a wavelength in free space at the highest frequency of operation. The size of the apertures of the grating will be typically tapered and vary in different regions of the grating to optimize the amplitude distribution over the antenna radiating surface.

The artificial dielectric structure has multiple periodically distributed via holes on a dielectric substrate which are plated and connected to top metallic grating layer. The via holes are not connected to the bottom ground plane, as the metal surrounding each via hole is removed. A secondary thin dielectric layer (Typically thickness in the range 50-1 25 microns) is bonded to the back of the bottom ground plane, as shown in Figure 2. Metal patches are printed in the thin dielectric outer face to form lumped capacitors with the bottom ground plane.

These built-in capacitors are placed around each via hole and connected to them by a switching device (such as a PIN diode). DC bias for control of the switches is provided as a voltage between the top ground plane and the printed capacitors which arc connected to the DC source by an inductive line (often referred to as decoupling stubs). When the capacitors are switched on there is an increase the capacitance per volume seen by the electric field component parallel to the via hole. This leads to an increased effective dielectric constant in that direction. The dielectric constant for orthogonal directions remains relatively similar to the dielectric constant of the dielectric substrate.

It will be appreciated that other RF switching devices, for example PIN diodes or MEMS could be used, as shown in Figure 3. Tunable capacitors based on liquid crystals ([C), ferroelectric materials and semiconductors (i.e. varactors) can be used to replace directly the RF switch and fixed capacitor, although their relatively low range of capacitance will limit the span of effective dielectric constant so will not be appropriate for all applications.

A parallel resonator using a tunable capacitor can be set to a high or low impedance state as a narrowband replacement of a RF switch, as shown in Figure 4, which can be printed in large numbers (for example using liquid crystal technology). Another example could use light controlled switches to connect the loading capacitors to the transmission line (through the via hole). These optical switches can operate directly at RF (by using photodiodes) or act upon the DC bias of the RF switches or tunable resonators. This example can then produce a light controlled artificial dielectric slab. The desired dielectric constant across the overall aperture will be modified by a light pattern produced using conventional methods, such as an LCD display.

With the invention, there is a need of course a large number of switches that can turn on certain region (similar to a pixel in displays). The spacing between switches is much smaller than the wavelength (i.e. one tenth) therefore the number of switches is much larger than the control elements for a prior art phased array for the same aperture size. However, the complexity of the switches is much less than the phase control elements for a prior art device and they can be produced using printing techniques capable to produce them in larger numbers at low cost in a way similar to flat liquid crystal displays. This is also possible because switching elements do not dissipate a substantial amount of power and no associated amplifiers are required.

The lattice of via holes can have one of a number of configurations. Figure 5 shows hexagonal lattice that is smaller than the shortest wavelength considered inside the dielectric at the highest effective dielectric constant (typically in the range of one eighth or tenth of the wavelength). This large number of elements ensures a behavior similar to a continuous dielectric. At the same time, there are enough degrees of freedom to point a radiated beam to the desired direction with high resolution. The number of controls is quite large, typically thousands of them are required for a medium size antenna. However, the control devices are simple switches which operate directly in a digital form. Furthermore loss caused by the switching device is relatively small because the impedance of the capacitors is much higher than the resistance of the switch connected in series.

It has boon determined that, typically, the effective refractive index should be varied between 1.5 and 3.5 to scan a beam from -9Odeg to +9Odeg with respect to boresight.

There are several possibilities for the structure used for guiding the waves inside the dielectric slab. The simplest structure is a parallel plate waveguide allowing a TEM wave to propagate freely in between the ground planes. A second example of the antenna, makes use of parallel striplines supporting a set of TEM waves (Figure 6). A third example has a set of rectangular waveguides driving a TE1O formed in the artificial dielectric structure by shorting via holes. The variation of the dielectric constant of the arlificial material will change the propagation constant of the guided wave which will interact with the apertures on the top ground plane to produce a directive beam which will scan in the plane of the propagation of the guided wave (x direction).

All the possible configurations need a launching structure which illuminates the aperture and allows the scanning of the radiating beam in the plane perpendicular to the propagation of the guided wave (y direction). One preferred structure has a single TEM line filled with the controllable artificial dielectric which leaks power at periodic intervals into the main structure. In the case of a stripline, the leakage will be achieved by direct connection of the main set of lines to the launching one. In a parallel plate or waveguide implementation the leakage will be obtained by periodic apertures on the wall leaking into the main cavity.

Another embodiment, shown in figure 7 uses two TEM lines at opposite ends of the aperture, the lines feeding interleaved sets of parallel waveguides or transmission lines. This allows the generation of two separate scanning beams (one per feeding line at the edge of the aperture) which can be tuned for different frequency bands. This configuration is intended for dual band operation compatible with receive and transmit from the same physical aperture.

Additionally, one set of parallel waveguides or transmission lines will interact with elements of the gratings oriented towards a particular direction (i.e.slots) while the other set interact with elements on the orthogonal direction leading to two orthogonaly linearly polarized beams.

From the above it will be appreciated that the antenna of the invention has high switching speeds yet is still able to provide good directional control.

Furthermore, it can be formed by manufacturing techniques used in other technologies, such as for the manufacture of liquid crystal planar displays,that are low in cost and effective for mass production. Furthermore, by enabling the use of feeding striplines from different sources it is possible to employ a single antenna according to the invention for different frequencies allowing both transmission and reception by a single device.

Claims

CLAIMS1. A beam scanning antenna arranged to generate and aim a directive radiation beam, the antenna comprising: a waveguide comprising a controlled artificial dielectric structure comprising periodically distributed capacitors placed on a conductive wall of the waveguide; a fixed metallic grating layer on the wall of the waveguide; wherein the artificial dielectric is connected to the grating layer by plural via holes and respective electronic switching devices aligned with respective apertures in the grating layer such that, in use, the dielectric constant of the artificial dielectric layer can be controlled by the switches to control the beam direction.
2. An antenna according to claim 1,wherein the waveguide comprises a plurality of waveguides parallel to each other and sharing the fixed metallic grating as common wall to define a rectangular radiating aperture.
3. An antenna according to claim 2, further comprising a feeding waveguide filled with artificial dielectric and arranged perpendicular to the plurality of parallel waveguides and configured to feed an electromagnetic signal to the plurality of parallel waveguides by means of apertures or metallic probes.
4. An antenna according to any of claims 2 to 4, further comprising a fixed grating layer comprising interleaved rotated slots at right angles to one another and wherein the plural waveguides are arranged in two sets, a first set of waveguides arranged to interact with slot in a first orientation while a second set of waveguides are arranged to interact with slots in a second orientation in a direction orthogonal to the first orientation to provide dual polarization operation ofthe antenna.
5. An antenna according to claim 5 where one set of waveguides operate at one frequency band and the other set at different frequency band allowing dual band operation.
6. An antenna according to any of claims 2 to 4, with a curved grating and waveguide structure where the pitch of the grating is varied across the surface to radiate a planar phase front leading to a directive beam.
7. An antenna according to any of claims I to 5, wherein there is a tapered distribution of the apertures of the grating.